CVE-2020-28200 in Dovecot
Summary
by MITRE • 06/28/2021
The Sieve engine in Dovecot before 2.3.15 allows Uncontrolled Resource Consumption, as demonstrated by a situation with a complex regular expression for the regex extension.
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Analysis
by VulDB Data Team • 07/03/2021
The vulnerability identified as CVE-2020-28200 represents a critical resource consumption issue within the Sieve engine of Dovecot email server software. This flaw affects versions prior to 2.3.15 and specifically targets the regex extension functionality that enables users to create complex pattern matching rules for email filtering and routing. The vulnerability stems from inadequate input validation and resource management within the regular expression processing component of the Sieve engine, creating a potential denial of service condition that can be exploited by malicious actors.
The technical implementation of this vulnerability involves the exploitation of a resource exhaustion attack through malformed or overly complex regular expressions. When the Sieve engine processes such expressions, it fails to properly limit the computational resources required for pattern matching operations, leading to excessive cpu and memory consumption. This behavior aligns with CWE-400, which categorizes uncontrolled resource consumption as a fundamental weakness in software design. The vulnerability is particularly dangerous because it can be triggered through legitimate Sieve script execution, making it difficult to distinguish between normal usage and malicious exploitation. Attackers can craft specially designed regular expressions that cause the engine to enter into computationally expensive matching operations that consume disproportionate system resources.
The operational impact of this vulnerability extends beyond simple denial of service conditions, as it can severely compromise the availability and performance of email services. When exploited, the vulnerability can cause Dovecot processes to consume excessive system resources, potentially leading to system crashes, service degradation, or complete unavailability of email services. This affects organizations that rely heavily on email infrastructure for business operations, as email availability is often critical for communication and operational continuity. The vulnerability can be particularly damaging in high-volume email environments where multiple users may be simultaneously affected, potentially causing cascading failures across the email infrastructure. Organizations using Sieve-based filtering systems are particularly at risk, as these systems are commonly deployed in enterprise email environments where complex filtering rules are essential for proper email management.
Mitigation strategies for CVE-2020-28200 focus primarily on immediate software updates and configuration hardening. The most effective solution involves upgrading to Dovecot version 2.3.15 or later, which includes patches specifically designed to address the resource consumption issues in the regex extension. Organizations should also implement strict input validation measures for Sieve scripts, particularly those involving regex patterns, to prevent the execution of overly complex expressions. Configuration changes can include setting limits on regex complexity, implementing resource quotas for Sieve processing, and establishing monitoring systems to detect unusual resource consumption patterns. From an operational security perspective, organizations should conduct thorough testing of updated configurations to ensure that legitimate email filtering functionality remains intact while preventing exploitation of this vulnerability. The ATT&CK framework categorizes this vulnerability under privilege escalation and denial of service tactics, highlighting the need for comprehensive security controls that address both the immediate exploitation vector and broader system resilience considerations.